The present invention relates to the field of materials characterization, and in particular, to enhancing the yield of secondary ions for mass spectrometric analysis.
Focused ion beam (FIB) systems are widely used in semiconductor manufacturing because of their ability to image, etch, deposit, and analyze with extremely fine resolution. Secondary Ion Mass Spectrometry (SIMS) is a method, often used in conjunction with FIB systems, for determining the composition of a sample. In the SIMS process, the ion beam is used to sputter, that is, physically eject by energy transfer, particles such as atoms, molecules, and clusters, from the sample. A small percentage of these ejected particles are ionized, that is, they gain or lose one or more electrons during the sputtering process and are thereby electrically charged. Such ejected charged particles are known as secondary ions, as opposed to the primary ions in the focused ion beam. By passing the secondary ions through a mass spectrometer that includes a combination of magnetic and/or electric fields, it is possible to determine the charge-to-mass ratio of the secondary ions and then to deduce their composition.
Ideally, an analyst can detect the presence of a very small amount of a substance and pinpoint exactly where on a specimen the substance was located. The sensitivity of a SIMS is a measure of its ability to detect small quantities of a substance. Because the features size in modern integrated circuits is often as small as 0.12 micron, a very small contaminant can ruin a circuit. Thus, being able to characterize precisely a small amount of contaminant at a precise location is critical to determine the causes of circuit defects and to characterize circuit fabrication processes.
The ability of a SIMS to detect the presence of a material depends on the type of material being analyzed, the concentration of that material in the area being sputtered, the total amount of material sputtered from the sample, and the probability that the material sought will be ejected as a detectable ion and not as an undetectable neutral particle. Unfortunately, most of the particles sputtered from a specimen are not ionized and cannot be analyzed in the mass spectrometer, which uses electric and/or magnetic fields to separate charged particles.
The probability of a particle being ejected as an ion depends upon the elemental composition of the particle, the chemical environment around the particle on the sample, and the type of ions in the primary ion beam. Primary beams of oxygen or cesium ions are used for SIMS analysis with satisfactory secondary ion yields. Such beams, however, typically have a spot size of greater than one micron and cannot provide the extremely fine resolution of liquid metal source ion beams. Gallium liquid metal ion source (LMIS) primary ion columns, for example, can provide 5 nm to 7 nm lateral resolution on commercially available FIB systems. Because of their ability to image, sputter, and deposit with such great precision, FIB systems have gained nearly universal acceptance as a necessary analytical tool for semiconductor design, process development, failure analysis, and most recently, defect characterization. In support of these applications, the addition of a SIMS to the FIB has expanded the applications of the tool to include elemental analysis having sub-micron spatial resolution.
The smaller spot size of the gallium beam necessitates fewer ions in the beam, that is, reduced beam current, so the total number of ejected particles available for detection is correspondingly reduced. Moreover, the secondary ion yield, that is, the number of secondary ions ejected for each primary ion, from a gallium source can be a factor of 100 less than that of an oxygen primary beam. Thus, although the smaller gallium beam provides much greater precision in determining the position from which a particle is ejected, the total number of ions ejected is reduced and the probability of the particles that are ejected being ionized and detected is greatly reduced. Thus, SIMS used with LMIS is not as effective as analysts desire.
The yield of secondary ions also depends on the chemistry of the specimen. It is known that if a surface is oxidized the percentage of positive ions produced during the sputtering process increases compared to the number for an unoxidized surface. Most metals and semiconductors are more likely to lose an electron than to gain one and are therefore more likely to become positive ions than negative ones. Thus, the SIMS sensitivity for metals and semiconductors from an oxidized surface is increased. It has therefore become common practice to introduce oxygen as a gas into the vacuum system in order to oxidize the surface. It is also known to use an oxygen ion primary beam, with or without oxygen gas introduction, to enhance secondary ion yield. Using an oxygen ion beam, however, is not possible in the majority of modem FIB systems, which are designed to use a LMIS and, as described above, oxygen primary ion beams lack the spatial resolution to be useful in many applications.
Thus, it is an object of the invention to increase the sensitivity of SIMS systems.
It is another object of the invention to increase the sensitivity of SIMS systems used with a sub-micron spot size LMIS FIB system.
It is a further object of the invention to facilitate elemental analysis of sub-micron features using SIMS.
It is yet another object of the invention to enhance detection of metallic and semiconductor materials using SIMS.
It is still another object of the invention to increase the secondary ion yield using a safe, non-toxic agent.
It is yet a further object of the invention to introduce an agent onto the target surface while minimizing the reduction in secondary ion collection efficiency reduction caused by the agent introduction apparatus.
It is still a further object of the invention to minimize the disruption of the secondary ion extraction field caused by the agent introduction apparatus.
It is yet a further object of the present invention to improve the sensitivity of SIMS in existing LMIS FIB without requiring expensive system modifications.
In accordance with the invention, water is introduced onto the surface of the specimen in an area that is being sputtered for SIMS. The introduction of water has been found to result in a significant enhancement of the secondary ion yield for most materials, including silicon, aluminum, titanium, molybdenum, and tungsten.
The water is introduced preferably using a gas nozzle near the target point of the primary beam, so that the water molecules have a high probability of sticking to the surface from which particles are sputtered. To prevent the close proximity of the nozzle to the surface from adversely affecting the secondary ion extraction field and degrading the collection efficiency, the nozzle may be electrically biased.
The present invention provides improved SIMS sensitivity on existing LMIS FIB systems that are in common use in the semiconductor and other industries. Enhancement of secondary ion yields significantly improves detection sensitivity and thereby facilitates elemental analysis of smaller (sub-100 nm) features.
Additional objects, advantages and novel features of the invention will become apparent from the detailed description and drawings of the invention.